Formation Entropy of Schottky Defects and Free Formation Enthalpy of Vacancy Pairs in Alkali Halide Crystals

The melting of alkali halides occurs probably at a critical mole fraction x_c = 3.2 · 10⁻⁴ of thermally created vacancies (Schottky defects). Using this value the formation entropy of Schottky defects is found to be s_S = (962.1 K mol/kJ L/T_m – 16.1) k (L, T_m heat and temperature of fusion, respectively). As to the free association enthalpy g_A = h_A – T_sA of (impurity-vacancy) complexes it is s_A = (10.8 h_A/eV – 2.56) k. Using this relation the concentration vacancy pairs is found to be smaller than that of isolated vacancies up to the melting point.     

The influence of the Mg content and the quenching conditions on the decomposition kinetics of Al?Zn?Mg alloys

By means of resistivity measurements, TEM and microhardness measurements the decomposition kinetics of A1-3 at.% Zn-1.5 at.% Mg and A1-4.5 at.% Zn-x at.% Mg (x = 0.2; 0.5; 1.0; 1.5; 2.25; 3.0) were investigated. For T_a < T_{a, crit} < T_h (T_h: temperature of the rapid homogenous nucleation) in A1-3 at.% Zn-1.5 at.% Mg and A1-4.5 at.% Zn-x at.% Mg (x = 1.0; 1.5; 2.25; 3.0) an inversion of the decomposition kinetics for T_q > T_inv: (T_inv: inversion temperature) was established. This result can be explained with reference to the binding of excess vacancies in dislocation loops. Proceeding from the discrepancy between the experimentally estimated concentrations of vacancies bound in dislocation loops and the concentrations of free vacancies at T_q, calculated by means of the Lomer equation, a model is proposed, which explains the high vacancy concentration of the loops. In terms of this model the values of the migration energy, obtained for A1-3 at.% Zn — 1.5 at.% Mg in the T_a-range 23 °C ≦ T_a ≦ 100 °C, and the shift of the inversion temperatures T_inv towards higher values with increasing T_a are interpreted. The same applies to the influence of the Mg content on the decomposition kinetics in A1-4.5 at.% Zn-x at.% Mg alloys.     

Structural Changes during Post-Ageing of an Al-Zn (15 at.%) Alloy between 125 °C and 215 °C

The structural changes going on in an Al Zn (15 at.%) alloy postaged between 125 °C and 215 °C after pre-ageing between 20 °C and 90 °C (till start r_s = (20…︁ 55) Å are obtained) were followed by means of XSAS-investigations. The main results are the following:

• 1 An increase of r_s does not favour the growth of the precipitates during the reversion treatment.
• 2 At later reversion times the happening is obviously independent of the pre-history of the samples.
• 3 The integral intensity is decreasing with increase of T_rev, but the mean distance (d)-size (r_G) relation between the precipitates is not changed remarkably up to T_rev ≈ 200 °C in the early reversion stage.

Conclusions from this behaviour are drawn.     

Investigations on the growth kinetics of GP zones and a′R-precipitates in Al-Zn alloys

X-ray small-angle scattering (XSAS) and resistivity (R) measurements were particularly done with an Al-Zn (15 at·,%) alloy in rather wide ranges of both the ageing time t_a and ageing temperature T_a, in order to obtain information on the dependence of the growth exponent m of the l = β₀t^m growth law and the activation energy E_act on t_a and T_a. The XSAS-measurements yielded that within the range of the GUINIER radius r_G between 1 nm and 2 nm the growth is essentially retarded (m < 0.1) and for r_G > 2 nm m depends on T_a ranging from 0.15 to 0.23 with a maximum at 175°C. Reasons for these effects are discussed. The differences between the m-values obtained by means of XSAS-and TEM-measurements are explained by the distinctions of the two methods applied. The E_act taken from XSAS- and R-measurements show a remarkable increase with t_a. At the beginning of the decomposition E_act = (0.49 ± 0.05) eV holds well explainable by the migration of quenched-in VZn pairs, but at the end E_act = (1.05 ± 0.07) eV was found. This value was also obtained from TEM-investigations (growth of the length). It fits well the E_act of ZnV pairs in thermal equilibrium at T_a.     

A Mechanism of High-Tc Superconductivity Relating to Perovskite Crystal Structure

A mechanism of high-T_c superconductivity related to the perovskite crystal structure is proposed by a “phonon attenuation model”, which means that in the perovskite-like crystal structure, a number of “ordering” vacancies or holes occur due to the non-stoichiometric structure features. Such a reticular “spongly-like” structure facilitates lattice thermovibration attenuation perovskite (phonon attenuation) during temperature reduction. This leads to an abrupt drop of the resistivity, owing to the conduction electrons transmitted through the media without collision with irregularities in the lattice due to the lattice thermovibration. This probably is the main reason for high-T_c superconductivity of the perovskite cuprates.     

Formation Volume of Vacancies in Elements and the Defect Formation Mechanism of Melting

The formation volume V_v of vacancies is given by V_v = (h_v/L) ΔV_f with h_v = 8L (formation enthalpy h_v of vacancies and heat L of fusion given in same units; ΔV_f = change of volume due to melting). If there are phase transitions within the solid, L and ΔV_f must be replaced by (L + Δ H_t) and by (ΔV_f + ΔV_t), respectively (Δ H_t and Δ V_t refer to the heat (s) of transition (s) and to the volume change(s) due to transition(s), resp.). The pressure dependence of the melting point is dT_m/dp = (T_mV_v)/h_v. Independent of the sign of V_v any increase of the vacancy concentration above the maximum concentration possible within the solid decreases the melting point thus resulting in the observed surface melting. The melting point is fixed by the characteristics of vacancy formation (h_v, S_v, V_v) and by the bulk modulus of the solid (S_v = formation entropy of vacancies).     

Estimation of point defect parameters of solids on the basis of a defect formation model of melting (III). Formation entropy and concentration of schottky defects (vacancies)

The formation entropy of Schottky defects (vacancies) is calculated from the entropy of fusion and the concentration x_L = 0.125 of vacancies within the melt at melting point T_m. The formation of vacancies is connected with a decrease of the vibrational frequency of those lattice forming particles (atoms, ions, molecules) being neighbours of the vacancies. Theoretical values of the vacancy concentrations x_s agree with those x_s obtained with the help of experimentally determined free formation enthalpies of the defects.     

Dependence of the Electric Properties and the Positron Lifetimes on the Dopant Content in La-doped SrTiO3

The resistivity, the dielectric constant, and the positron lifetimes in La-doped SrTiO₃ have been measured for La content x of 0—10 at.%. It was found that with increasing x, the variations of the resistivity and the dielectric constant and the positron lifetime parameters are nonmonotonic. The positron experiments have shown that the La-doping induces mainly formation of Sr vacancies and variation of Sr vacancy configuration; the most probable configuration is the isolated Sr vacancies (V) as x < 0.5 at.%, the associated defects (La V) for 0.5 < x < 1 at.%, and the associated defects (2 La V) above x = 1 at.%. The results suggest that the variation of the resistivity can be regarded as variation of electron density, and the variation of the dielectric constant results mainly from variations of the space-charge polarizaion and Sr-vacancy concentration and configuration.     

The influence of the direct quench and indirect quench to Ta > Th on the decomposition behaviour of an Al-2 at.% Zn-1 at.% Mg alloy

The decomposition behaviour of an Al-2.0 at.% Zn-1.0 at.% Mg alloy after direct quench (DQ) and indirect quench (IQ:T = 23 °C, t) to the ageing temperature T = 160 °C (above the temperature of the rapid homogeneous nucleation, T_h) was investigated by means of TEM and isothermal resistivity measurements. The precipitation density of the η-particles yields a maximum value at pre-ageing times about 10 min at T = 23 °C. To clear up the reason for that phenomenon a loop-formation and clustering model was introduced.     

Vacancy formation enthalpy in AgZn alloys

Effective vacancy formation enthalpy (H_v) for AgZn alloy was determined by means of positron annihilation method. It was found that H_v decreases linearly with increasing Zn content for the α-phase. For the ξ and β′-phase the H_v does not change and is equal to 0.53–0.55 eV.     

On the influence of impurity additions and quenching temperature on the course of the microhardness (HV) during a two-step ageing treatment of an Al-2.0 at.%Zn-1.3 at.%Mg Alloy

A pure as well as the analogous technical alloy were directly quenched from T_q = 400 °C or 490 °C to T_a, preaged at T_a (ranging between 50 °C and 160 °C) various times (t_a), and subsequently postaged at R.T. up to t_post = 42 d. — Ageing the samples below the upper limit temperature of the homogeneous formation of G.P. zones (T_hn) no essential difference in the course of the HV-number attained after one- as well as two-step ageing in dependence on the addition of impurity atoms and T_q could be found. — Contrary to this behaviour after pre-ageing at T_a > T_hn and postageing at R.T. a variation of both T_q and the content of impurity atoms shows a considerable effect. The reason is the difference in the density of nucleation sites for heterogeneous formation of precipitates of the η′- and/or η-phase in dependence of T_q and impurity content.     

An analysis of the applicability of the Scheil equation in the determination of segregation in monocrystals of the manganese-zinc ferrites Mn1?xZnxFe2O4

Theoretical assumptions concerning the segregation phenomenon of alloying components during formation of monocrystals of the manganese-zinc ferrite Mn_{1 x}Zn_xFe₂O₄ have been presented. Taking into account the case of the unidirectional solidification of the above ferrite by means of the Bridgman furnace, which is working in:

• Closed system.
• Open system, with adding of the pure component (melting at an elevated temperature).
• Open system, with adding of ferrite of a nominal composition.
• The method for calculation of alloying components segregation has been analysed. A possibility of modelling the segregation by means of the choice of the size of the melting zone has been considered for a given monocrystal.

     

Vacancy defects in as-grown and nº-irradiated GaP and GaAs1−xPx Studied by Positrons

Measurements of positron lifetime τ and of the shape parameter S of the Doppler-broadened annihilation line are used to study bulk and defect properties in GaP and GaAs_1−xP_x. τ and S decrease linearly with the composition x of GaAs_1−xP_x layers. A second lifetime component (τ₂ = 290 ps) observed in as-grown GaP is attributed to stoichiometric P vacancies. After neutron irradiation of GaP and GaAs_0.13P_0.87 positrons are trapped by Ga vacancies (τ₂ = 250 ps). These vacancies anneal out in two stages at 200–550 °C and 550–800 °C.     

Accurate Refinement of the Crystal Structure of Ba<Subscript>3</Subscript>TaGa<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>14</Subscript> from Langasite Family and Analysis of Structural Transformations Due to Isomorphous Cation Substitutions

An accurate structure analysis of a Ba₃TaGa₃Si₂O₁₄ single crystal from langasite family was performed using four X-ray diffraction data sets collected on a diffractometer equipped with a CCD area detector (sp. gr. P321, Z = 1, sinθ/λ ≤ 1.35 Å^–1; at 295 K a = 8.516(1) Å, c = 5.1910(6) Å, R/wR = 0.58/0.56%, Δρ_min/Δρ_max =–0.73/0.42 e/ų, 4414 independent reflections; at 106 K a = 8.5109(9) Å, c = 5.1861(9) Å, R/wR = 0.75/0.86%, Δρ_min/Δρ_max =–0.81/1.06 e/ų, 4382 reflections). The distinguishing feature of the Ba₃TaGa₃Si₂O₁₄ structure is a strong disorder of the Ga atom at the 3f site. Structural transformations in the series of Сa₃TaGa₃Si₂O₁₄–Sr₃TaGa₃Si₂O₁₄–Ba₃TaGa₃Si₂O₁₄–Ba₃TaFe₃Si₂O₁₄ crystals were analyzed.     

On the reversion behaviour of an Al-15 at.% Zn alloy between 155°C and 175°C

The structural changes going on in an Al-15 at.% Zn alloy postaged in the vicinity of the kink of the C-curve (161°C) in the T.T.T.-diagram after pre-ageing at R. T. or 90°C (till start radii r_S between 1.0 and 4.0 nm were obtained) were followed by means of SAXS-investigations. The main results are the following:

• 1 No indication could be found that the reversion behaviour is remarkably changed by crossing the temperature of the kink of the C-curve in the T.T.T.-diagram, i.e. the reversion as well as the growth mechanism of the zones does not differ considerably below and above 161°C.
• 2 The decrease of the integral intensity Q₀ during the reversion process is mainly due to both the loss of zinc content of the zones and its rise in the matrix and not essentially by the diminution of the volume fraction of the precipitates.
• 3 Zones of a start radius of about 4 nm are much more stable against the decrease of the zine content during the reversion treatment that those of 2 nm.

     

Low-temperature X-ray studies of a [(CH3)2NH2]2 · CuCl4 crystal

The a, b, c lattice parameters of a [(CH₃)₂NH₂]₂ · CuCl₄ crystal have been measured by the X-ray diffraction method within the temperature range of 100–300 K. The temperature dependences of thermal expansion coefficients α_a = f(T), α_b = f(T), and α_c = f(T) along the principal crystallographic axes and thermal expansion coefficient of the unit-cell volume αV = f(T) are determined. It is found that all the three parameters, a, b, and c, vary with temperature in a complicated way and show jumplike anomalies in the a = f(T), b = f(T), and c = f(T) curves at phase-transition temperatures T _c1 = 255 K and T _c2 = 279 K. An incommensurate phase with the modulation wave vector q _i = (1/2 + δ)(a* + c*) is revealed in the temperature range 279–296 K. It is shown that the incommensurability parameter δ increases with an increase in temperature.     

On the Influence of Short R.T. preageing on the formation of the η′-phase in an Al-Zn(4.5 at.%)-Mg(2.5 at.%) alloy

The influence of short R.T. preageing (1 min) after quenching from 480 °C into RT-water on the formation of the η′-phase in an Al-Zn(4.5)-Mg(2.5) alloys has been investigated by means of TEM, XSAS, resistivity, and hardness measurements in the ageing temperature range between T_a = 120 °C comparing samples directly quenched (DQ) to T_a and those quenched into RT-water, kept there for 1 min, and subsequently stored in an oil-bath of T_a (IQ). At T_a ≦ 170 °C both heat treatments result in finely distributed precipitates of the η′-phase accompanied by a fairly high hardness value. But at T_a ≧ 180 °C the DQ leads only to few coarse heterogeneously formed η′- and η-precipitates, whilst the IQ yields qualitatively the same precipitation structure as already described at T_a ≦ 170 °C even till 280 °C. These results are interpreted assuming that. A1) the limitation temperature of homogeneous formation of η′ is located at about T_h = 175°c and A2) during the pre-ageing at RT “nucleation sites” are formed as already suggested by RYUM, which are able to act as sites for heterogeneous formation of η′ even at T_a > T_h. Hints are obtained that the formation of these “nucleation sites” is not so strongly governed by the quenched-in excess vacancies as discussed in the literature.     

Structural study of new compound Bi<Subscript>2.53</Subscript>Li<Subscript>0.29</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript> by the powder neutron diffraction method

A new compound of composition Bi_2.53Li_0.29Nb₂O₉ was synthesized in the course of the search for new materials with high ionic conductivity. Its crystal structure was determined from the neutron diffraction data. The new compound Bi_2.53Li_0.29Nb₂O₉ is crystallized in the orthorhombic system, sp. gr. Cmc2₁, and unit-cell parameters a = 24.849(1) Å, b = 5.4536(3) Å, and c = 5.4619(2) Å at T = 290 K (a = 24.843(2) Å, b = 5.4456(5) Å, and c = 5.4546(5) Å at T = 10 K). Within the temperature range 10–870 K, no structural phase transitions were revealed. The atomic coordinates and the thermal factors in the isotropic approximation were refined by the Rietveld method at 290 and 10 K. The data obtained were analyzed based on the calculated local balance of bond strengths.     

Specific features of the crystal structure and magnetic properties of KTaO<Subscript>3</Subscript> produced by electrolysis of melts

The magnetic susceptibility χ(T) at 4.2 K < T < 293 K; the dependence of the magnetic moment on the magnetic field strength, M(H), at 4.2, 77, and 293 K; and the electrical resistivity ρ(T) at 4.2 K < T < 293 K are studied for samples of perovskite-phase KTaO₃ obtained by both solid-phase synthesis (KTaO ₃ ^s ) and deposition on a cathode during electrolysis of melts (KTaO ₃ ^e ). Yellowish white KTaO ₃ ^s powders are diamagnetic and reveal dielectric properties. Dark polycrystalline KTaO ₃ ^e samples with metallic luster are characterized by the dependence ρ(T) typical of metals and additional paramagnetic contribution to the paramagnetic susceptibility as compared with KTaO ₃ ^e . Changes in the properties of KTaO₃ during electrocrystallization are attributed to partial reduction of tantalum. They are revealed in the structural features of KTaO ₃ ^e (excess of tantalum as compared to the stoichiometric composition of KTaO ₃ ^e , deficiency of the oxygen sublattice, and clearly pronounced anharmonicity of atomic vibrations). A change of the cation-anion-cation interactions, occurring owing to the overlapping of oxygen p orbitals with tantalum t_2g orbitals and the formation of impurity levels near the conduction band, leads to the generation of free carriers, which make a paramagnetic contribution to the magnetic susceptibility.